The traffic capacity of radio networks is increased by dividing a geographic area to so-called cells and by using the same carrier frequencies simultaneously in different cells, as known. The capacity of a network is the higher the smaller the cells are and the closer to each other the cells are in which the same carrier frequencies can be used. Instead of an omnidirectional antenna, a plurality of antennas radiating controllably in different sectors are often used in the base stations of the cells. In that case the base stations at a certain distance from each other, using the same carrier frequency, interfere less with the transmitted signals of each other. This means that the reuse distance of frequencies can be reduced and the capacity of the network thus further increased.
Both the transmitting power and the direction of the transmitting in the vertical plane of an antenna radiating in a certain sector have to be chosen so that the coverage area is sufficient, but on the other hand the interfering influence in the neighboring cell is slight enough. The angle between the middle direction of the transmitting main lobe and the horizontal direction is called “tilt angle”. If no changes were to happen in the circumstances, the tilt angle would be constant without adjusting possibility. However, in practice the traffic intensity in the cells fluctuates a great deal. During minor traffic it is advantageous to keep the tilt angle smaller than during heavy traffic, because in that case the connection quality in the border regions of the cells becomes better without the total interference remarkably growing in the neighboring cells. In addition, the shape of the built environment in the cell can change so much that there is reason to change the tilt angle.
Changing the direction of the antenna radiation lobe, without turning the antenna mechanically, succeeds when an array of radiators is applied. When the phases of the carriers fed to the radiators in a row are arranged to have suitably different values, the lobe turns off into the desired direction from the normal of that row, as known. Changing the tilt angle then requires adjustable phase shifters in the feed paths of the radiators and that the radiators are located in a substantially vertical row. The radiator row can deviate from the vertical direction as much as a typical tilt angle is achieved without any phase shifts. After that the tilt angle can be changed upwards and downwards by means of phase shifts.
The phase shifts needed in the feed of an adjustable antenna are so great at the maximum that in practice only transmission line type solutions come into question as phase shifters. The physical length or at least the electric length of a transmission line has to be changeable by electric control. A wholly electric adjustable phase shifter is obtained, when the length of the transmission line is changed e.g. by means of diode switches or ferrite pieces being located in the space where the field propagates in the transmission line. In the latter case the permeability of the ferrite and thus the effective phase coefficient of the whole transmission line is changed. A disadvantage of these kinds of electric solutions is the losses caused by them, and in the case of diodes also the non-linearity. They are also expensive, if the phase shifters are made satisfactory for transmitting use by power capacity. Therefore the phase shifters used in the transmitters of base stations are in practice electromechanical so that they include a structural part movable by an actuator, the location of which part determines the (electric) length of the transmission line. In this description and claims such a structural part, movable along a line, is called “slide”.
A simple electromechanical phase shifter has a straight transmission line and a slide, by which a tapping is formed in the line. A radio frequency signal is fed to the line end and is taken out from the tapping. When e.g. a 225-degree phase shift is needed, the distance between the line end and the slide is adjusted to have value 0.625λ. λ is the wavelength in the line and it depends on the dielectricity and permeability of the medium between the line conductors. The length of the transmission line has to correspond directly to the greatest phase shift needed, of course. The length of the transmission line and thus the space required for the circuitry is reduced, when a reflection in the transmission line is utilized. In this case a short-circuit, and not a tapping, is formed in the transmission line by means of a movable slide. A signal, or electromagnetic field, arriving to the short point reflects to the reverse direction, as known. When the signal has arrived back to the starting end, it has traveled a double distance, for which reason also the phase shift is double compared to the structure, where the signal is taken out from the tapping being located at the same distance. For obtaining a certain maximum phase shift, a line having half length is then sufficient. That kind of shorted transmission line requires a separating element as an additional structure, which element separates the reflected signal, being in the same line with the incoming signal, to a transmission path of its own for feeding to the antenna. A circulator, for example, is suitable as such a separating element. A shorted line together with a circulator forms a phase shifter. More generally, in this description and claims a phase shifter using signal reflection includes also a separating element.
In this description and patent claims the term “reflection line” means a transmission line having in its tail end a circuit, which causes a reflection, so that a signal fed to the starting end comes also out from the starting end.
Using two parallel reflection lines and a four-port hybrid as a separating element instead of one reflection line and a circulator, a higher power capacity and better linearity are achieved. FIG. 1 shows an example of this kind of phase shifter suitable for the antenna feed circuit, known from the publication U.S. Pat. No. 6,333,683. The structure comprises a first reflection line 141, a second reflection line 142 and a hybrid 150, which has four ports P1-P4. The input line 101 of the structure is connected to the first port P1, and the output line 102 is connected to the fourth port P4. The first reflection line in turn is connected to the second port P2, and the second reflection line is connected to the third port P3. A radio frequency signal fed to the first port can propagate through the second and third ports to both reflection lines; there is 90-degree phase difference between those two partial signals. The reflected signal arriving to the second port from the first reflection line and the reflected signal arriving to the third port from the second reflection line have the same 90-degree phase difference, because the reflection lines are equal in length. Arriving to the first port of the hybrid, the reflected partial signals have opposite phases, and arriving to the fourth port they have the same phase. The reflected signal then can propagate only to the output line 102 through the fourth port P4. The input line, output line and reflection lines are all similar by structure. The cross section of the line structure as magnified is seen in the upper supplementary drawing in FIG. 1. Each line comprises two strip-like ground conductors GND one on top of the other and one narrower centre conductor CEC between the ground conductors. The medium is mostly air.
The reflection lines are located parallelly, and crosswise between them there is a shared dielectric slide 130. One end of the slide implements the short-circuit in the first reflection line 141 and the opposite end implements the short-circuit in the second reflection line 142. The slide fills in its location almost wholly the space between the ground conductors in both lines. For the centre conductor of each line the slide has a flat hole in the direction of the line. As can be seen, the short-circuit is not galvanic. The dielectric medium only enhances the capacitance between the centre conductor and ground conductors in the location of the slide so much that there prevails almost a short-circuit in the operating frequencies of the antenna.
Because of the structure described above the reflection lines become as much longer or shorter, when the slide 130 is moved. They are always equal in length, in which case the phase shifts always are equal in them. This is necessary in order to get the partial signals with the same phase to the fourth port of the hybrid 150 for summing and feeding to the antenna.
In FIG. 2 there is an example, known from the publication WO98/21779, on how to arrange the phase differences for the radiators of a group antenna to steer the radiating lobe. The antenna comprises three radiators, which are located in the same mast at different altitudes. The radio frequency signal IN coming from the power amplifier of the transmitter is divided into two parts by the divider 210. One part is led directly to the middle radiator. The other part is led to the phase shifter 200 and through it half and half to the uppermost radiator and to the lowest radiator. The phase shift structure differs from the structure according to FIG. 1. Its transmission line 220 has the shape of a circle arc, and the slide 230 is moved by a rotational motion. For this purpose the slide is located at the end of an arm 215, which has been provided with an axis to its opposite end. At the same time the arm functions as a feed line of the transmission line 220. The axis is rotated by an electric motor. The first end of the transmission line, or the first output of the phase shifter, is connected to said uppermost radiator, and the second end, or the second output of the phase shifter, is connected to said lowest radiator. When the slide is in its middle position, the signals of all three radiators are in the same phase, in which case the antenna main lobe is perpendicular to the straight line drawn along the radiators. When the slide 230 is located closer to the first end of the transmission line 220 than to its second end, the phase of the uppermost radiator leads the phase of the middle radiator, and the phase of the lowest radiator lags the phase of the middle radiator. In this case the antenna main lobe has been turned downwards from the above-mentioned perpendicular position. Correspondingly, when the slide is located closer to the second end of the transmission line than to its first end, the antenna main lobe has been turned upwards from the said perpendicular position.
The phase shifter according to FIG. 2 can be called differential, because moving the slide changes the phases of the two output signals equally, but to opposite directions. As appears from the description above, the reflection is not used in this phase shifter.
From the publication WO01/13459 is known an arrangement comprising more than one similar differential phase shifters as in the previous example. The transmission lines of the phase shifters have the same midpoint of the curvature, and their slides are moved by a common rotatable arm, which functions as an input line, at the same time.